This invention generally relates to modulated carrier communications networks and more specifically to networks with alternate data transmission paths.
Modulated carrier data communications networks convey data between a central site and one or more remote sites over telephone lines or other communications links. The central site typically comprises a digital data transmitter and receiver, such as a digital computer and its associated input/output control system. Each remote site also comprises a digital data transmitter and receiver, such as a peripheral unit.
At the central site, a modulator circuit receives the digital data from the central site transmitter and converts the data to appropriate modulated carrier signals for transmission over a data transmission path to a designated remote site. At the remote site, a demodulator circuit receives the incoming modulated carrier signal and converts it back to its original digital form for use by the data receiver. Similarly, a data transmitter at a remote site supplies data to its modulator for conversion to modulated carrier signals for transmission back to the central site where a demodulator converts the incoming modulated carrier signal back into digital form for the data receivers at the central site. Generally, a modulator circuit and a demodulator circuit at each site are packaged together in a "modem".
A single data transmitter and receiver at a central site may communicate with one or more remote sites, or "drops", over one or more separate communications links. Each communications link couples a central modem and a number of drops. Appropriate circuits at the central site and each remote site enable any one remote site to communicate with the central site. Once a particular drop is identified, no other drops on the link are enabled to respond for the duration of the communication.
In a typical communications network, the central site is connected to each remote site over a link or path comprising two pairs or sets of transmission lines to enable "full-duplex" operation. One pair carries modulated carrier signals to a remote site while the other carries signals from the remote site. These lines constitute primary data transmission paths. They are not merely two pairs of conductors. Rather they include switching elements, amplifier elements and other circuit elements as well as conductor elements that are common to telephone lines. A failure of any one of these elements can produce a primary data transmission path malfunction.
If the primary data transmission path constitutes the only connection between a remote site and a central site, a malfunction in the path terminates all communications with that remote site. Even if only one pair of transmission lines is malfunctioning, it normally is not possible to switch the modem from full duplex to "half-duplex" communications so all communications pass over one pair of lines due to the construction of the transmission lines.
There are available means for establishing a secondary, or back-up, data transmission path. Manually operated switching units are located at the central and remote sites in these networks. With these switching units, however, personnel at the remote site also must act to effect a change. Typically, these people are not technically trained, so personnel at the central site must coordinate the procedures for shifting between the primary and secondary paths. With these manual switching units, an additional data path (e.g., a third telephone line) must be used to coordinate the change over.
With manual switching units the network is operating exclusively over either the primary data transmission path or the secondary data transmission path. After a fault is ostensibly corrected, the secondary path is disconnected and communications are re-established over the primary data transmission path. If the primary data transmission path still malfunctions, the entire manual transfer procedure must be performed again.
Therefore, it is an object of this invention to provide at a remote site automatic switching between primary and secondary data transmission paths.
Another object of this invention is to provide an automatic switching unit which eliminates the need for any intervention by personnel at the remote site.
Another object of this invention is to provide an automatic switching unit which enables communications over both the primary and secondary data transmission paths simultaneously.